1. Field of the Invention
The present invention relates to a flow sensor for measuring a fluid flow rate. In particular, the present invention relates to a flow passage structure of the flow sensor.
2. Description of the Related Art
A flow sensor (flow meter) has been previously suggested, which has a flow passage for allowing a fluid to pass therethrough, and wherein a sensor is installed that faces the flow passage, in order to measure the flow rate of a fluid including, for example, air or nitrogen.
Japanese Laid-Open Patent Publication No. 2005-315788 discloses a flow sensor, comprising a flow passage that is formed by enclosing a groove having an open upper portion with a lid member. A sensor is installed so as to face the flow passage, and each of two meshes (mesh members) are provided at an inlet portion (upstream side) and at an outlet portion (downstream side) of the flow passage. In the flow sensor, measurement accuracy is improved by adjusting the flow rate of the fluid that flows through the flow passage by means of the meshes.
However, in the flow sensor constructed as described above, it is necessary for a plurality of meshes to be installed, in order to maintain the measurement accuracy of the flow rate measured by the sensor. Further, the flow rate of the fluid is subjected to turbulence, due to collisions against the mesh on the downstream side, which is disposed just after the other mesh. Therefore, it is necessary to provide a predetermined distance (length) between the mesh and the sensor, so that turbulence of the fluid can be converged. Therefore, there is a tendency for the flow passage to be prolonged. As a result, the structure of the flow sensor becomes large in size overall. Further, even when dust, dirt or the like becomes adhered to the mesh, it is impossible to exchange the mesh easily. When the mesh is exchanged, it is necessary to perform adjustments again, since the measurement accuracy of the flow sensor fluctuates.
On the other hand, Japanese Laid-Open Patent Publication No. 2004-3887 and Japanese Laid-Open Patent Publication No. 2004-3883 respectively disclose flow sensors that do not use meshes therein, unlike the aforementioned structure, in order to minimize the size of the structure. In such a flow sensor, the flow passage portion where the sensor is positioned is constructed so as to be substantially perpendicular to the flow passage portion disposed upstream therefrom. Further, a circular arc-shaped surface, which protrudes into the flow passage, is formed on at least a part of a wall that defines the flow passage portion where the sensor is positioned. Thus, the flow rate of the fluid can be adjusted without providing any meshes therein.
The flow sensor constructed as described above has a structure so that foreign matter, such as dust, dirt or the like, is removed by the wall surface of the flow passage, which is constructed substantially perpendicularly as described above. However, it is difficult to remove foreign matter when the flow rate of the fluid is large, for example. Further, when the circular arc-shaped surface is formed in a portion of the flow passage where the sensor is positioned, and if fluid having a large flow rate suddenly flows thereinto, then the fluid is lifted upwardly by the circular arc-shaped surface and abuts against the sensor, causing an impact. Thus, depending on the conditions of use, is feared that the sensor may become damaged.